1. Field of the Invention
The present invention relates to a sheet conveying apparatus for supplying an image forming apparatus with a sheet, and also to a skew correction of a conveyed sheet and a sheet alignment in an image forming portion of an image reading portion.
2. Related Background Art
In an image forming apparatus such as a copying apparatus, a printer or a facsimile apparatus, a sheet conveying apparatus is provided for conveying a sheet such as a recording sheet or an original to an image forming portion or an image reading portion. Some of such sheet conveying apparatus is known to have correction means for correcting a skewed conveying of the sheet in order to rectify a posture and a position of the sheet until it is conveyed to an image forming portion or an image reading portion.
As correcting method for such correcting means, there is known so-called loop registration method utilizing a pair of registration rollers, in which, for example in an image forming apparatus, a front end (leading edge) of a sheet is made to impinge on a nip of stopped registration rollers to form a loop in the sheet, whereby the front end of the sheet is made to be aligned along the nip of the rollers by the elasticity of the roller to correct the skewed conveying, and the registration rollers are thereafter rotated at a predetermined timing to match the front end of the sheet with a front end of an image.
In such loop registration method, however, there is necessitated a loop space for forming a loop, thereby making the apparatus inevitably bulky. Also in case a sufficient loop space cannot be secured, there may result difficulties of generating a jamming (sheet clogging) by a sheet buckling particularly in a sheet of low rigidity such as a thin paper, and generating a noise (so-called loop noise) when the sheet impinges on the registration rollers.
There is also a difficulty that the skew correcting ability varies depending on the rigidity of the sheet. More specifically, in a thin paper with a low rigidity, a contact pressure when the front end of the sheet abuts against the nip of the registration rollers becomes deficient and the front end of the sheet may be unable to sufficiently abut against the paired registration rollers, whereby a complete skew correction is impossible to achieve.
Also in a thick paper of a high rigidity, there may result a difficulty that the sheet penetrates through the nip of the paired registration rollers by an impact at the abutting against the registration rollers, and a measure for example of applying a load or the like to the paired registration rollers for example by a braking member leads to an increase in the cost of the product.
Furthermore, in case of a curling or a dog-ear at the front end of the sheet, the front end of the sheet cannot be exactly aligned along the nip of the paired registration rollers, whereby the skew correction cannot be executed precisely thus deteriorating the precision of printing.
On the other hand, the image forming apparatus and the image reading apparatus have recently become capable, by a digital system, after reading an original, of storing image information thereof as electrical codes in a memory portion. Also at the image formation, the information in the memory portion is read and used for forming an image corresponding to the image information of the original on a photosensitive member by an exposure apparatus such as a laser or an LED array, so that a mechanical movement of an optical apparatus or the like can be dispensed with even in a copying operation of plural sheets.
It is thus rendered possible to shorten a gap between sheets, or a sheet interval, and to process many sheets within a short period. As a result, it is becoming possible, for example at the image formation in an image forming apparatus, to increase the practical image forming speed without increasing the process speed.
However, in case of employing a sheet conveying apparatus of the aforementioned loop registration method, the sheet interval is inevitably determined as the sheet is stopped once for forming a loop, thereby seriously affecting the improvement of the image forming speed (productivity).
In order to avoid such difficulty, Japanese Patent Application Laid-open No. H10-067448 proposes a sheet conveying apparatus employing a registration method capable of automatically correcting a skewed sheet conveying.
This sheet conveying apparatus is provided with a pair of conveying rollers (registration rollers) for nipping and conveying a sheet, a sensor for detecting a skew amount of the sheet provided at a downstream side of the conveying rollers in a conveying direction thereof, and skew correcting means for conveying rollers, which inclines the conveying rollers in a direction perpendicular to the sheet conveying direction, and corrects a skewed conveying of the sheet by displacing the conveying roller according to the skew of the sheet, based on information from the skew detecting sensor.
In a prior image forming apparatus, when a toner image formed in an image forming portion (photosensitive drum) is transferred onto a sheet, the sheet is advanced to a transfer portion at a predetermined timing in order to align the front end of the image in the transfer portion with the front end of the sheet, namely in order to synchronize the front end of the sheet with the toner image. Also in a prior image reading apparatus, the sheet alignment in an image reading portion is achieved by advancing an original to the image reading portion at a predetermined timing.
For advancing a sheet at a predetermined timing, there may be employed a method of employing the aforementioned registration rollers or a method of detecting a front end of the sheet by a sensor provided in a sheet conveying path and advancing the sheet based on a signal of such sensor. However, as explained before, the method employing the registration rollers cannot attain a sufficient precision because it is affected by the rigidity of the sheet.
On the other hand, the method of advancing the sheet based on the signal from the sensor has a high precision, but, in the aforementioned method of correcting the skewed conveying of the sheet by displacing the conveying roller, since the front end position of the sheet is changed by the skew correction, it is necessary to detect the front end of the sheet after the correction for achieving an exact detection of the front end position of the sheet.
For this reason, the position of the sensor is restricted to a position capable of detecting the front end of the sheet after correction of skew. However such restricted position of the sensor not only complicates the sheet conveying apparatus but also increases the distance from the sheet conveying apparatus to the image forming portion or the image reading portion, so that the image forming apparatus or the image reading apparatus provided with the sheet conveying apparatus is difficult to realize in a compact configuration and becomes costly.